1. Field of the Invention
The invention pertains to the field of optical source equipment and related methods for use in fiber optic communications. More specifically, the optical source equipment includes an optical source bank that is used, for example, in testing optical amplifiers and wavelength division multiplexing (WDM) systems.
2. Statement of the Problem
Rapid advances in WDM or dense wavelength multiplexing (DWDM) provide cost-effective increases in the capacity of fiber-optic data transmission systems through the use of multiple polarization orientations and multiple wavelengths of light. DWDM or dense wave division multiplexing is a higher-capacity version of WDM. WDM systems support the multiplexing of up to four channels or wavelengths on a single fiber. Commercially available DWDM systems support up to 40 wavelengths or channels, and this capacity is steadily increasing. Data transmission capacity is also increased by time division multiplexing (TDM) rates in which a plurality of separate data signals are transmitted on the same line. Maximum transmission capacity is determined as a trade-off between the DWDM channel count and the maximum supported TDM switching rate. For example, a system operating on 40 channels at OC-48 with TDM might operate at a net throughput of about 100 Gbps. Comparable future systems operating on the OC-192 protocol at 40 channels might have a net throughput of 400 Gbps, and a future system operating on 100 channels might have a net throughput of one terabit per second.
While these future systems do not yet exist, at least in terms of practical implementations, continuing advances in DWDM and TDM technology are expected to expand maximum net throughput rates over the next several years.
The advances in net throughput rates require corresponding advances in other fiber-optic system components, especially in test equipment and signal amplifiers. For example, erbium-doped fiber amplifiers (EDFA) are used in combination with DWDM systems to eliminate or minimize the use of regenerative repeaters, and can be used as in-line repeating amplifiers, transmitter booster amplifiers, and receiver pre-amplifiers. EDFA devices comprise a section of glass fiber, which is doped with erbium. Light pulsing through this section of fiber excites the erbium which, in turn, amplifies the light pulse. EDFA technology has been used, by way of example, to support a mix of four 2.5 Gbps digital video streams in delivery of an 80 channel AM cable television network signal over a 100 km distance using one EDFA at the output node and one at midspan.
Optical test equipment for use in testing system components for fiber-optic transmissions is being continually outdated in the face of rapid capacity advances. Traditional DWDM test equipment uses an eight or sixteen channel multiplexer where, for example, eight channels may be allocated to a mainframe. Attempts to provide additional sources; e.g., more than 40 laser diodes that are each linked with a corresponding channel, source modulation electronics, attenuator, polarization control, and error injection devices, produce unwieldy agglomerated test systems that are connected with a patchwork of optical cables. The test systems grow to occupy large amounts of space, and test measurement errors may be induced, for example, by movements in the optical cables that interconnect the respective devices. Depreciation and use of optical test equipment may comprise a substantial percentage, e.g., twenty or thirty percent, of the total manufacturing costs of optical system components.
With increasing densification of source bank arrays, it becomes overly expensive and complicated to provide a separate function generator for each array.
The present invention overcomes the problems that are outlined above by providing an improved modulation control system for use in optical test equipment and optical data transmission systems where there is no need to provide a separate function generator to drive each channel. This advantage is implemented through use of a rail system where each rail may be used to drive a plurality of channels based upon output from a single function generator.
The source modulation system broadly comprises a plurality of laser source channels each including a laser source card having a laser source. A modulation controller includes a plurality of function generators that are each capable of generating waveforms for use at the laser source channels. A number of rail lines connect the modulation controller with each of the laser source cards. The number of rail lines have a one-to one correspondence with the function generators. The laser source channels include a programmably controllable rail selection switch for use in switching between selected rail lines to provide a selected laser source drive input corresponding to a selected rail line. The waveforms generated on the number of rail lines are preferably selected to include at least two members of the group consisting of square waves, sawtooth waves, and sine waves.
The laser source card contains a programmably configurable switch for use in accepting a selected one of the waveforms as drive input for the laser source. The laser source card also preferably includes a gain block that is programmably configurable to adjust an amplitude of the waveform from the selected rail line, e.g., by attenuation.
The modulation controller preferably includes a number of waveform input connectors allocated to selected ones of the rail lines. Each waveform input connector is capable of receiving waveform input from an external function generator when an external function generator is connected to the waveform input connector. Each waveform input connector is also capable of providing the waveform input as output comprising an external waveform output. A corresponding number of programmably configurable waveform selection switches on the modulation controller are capable of selecting inputs between the corresponding function generator output and the external waveform output, each of the corresponding number of switches being allocated to one of the selected ones of the rail lines.
The modulation controller may also include a coherence rail system having a coherence control function generator capable of generating a coherence control waveform output and a programmably controllable coherence rail switch capable of selecting between the coherence control waveform output and a ground. In this case, the programmably controllable rail selection switch in each laser source card is capable of selecting between the coherence rail system and the shared rail systems to provide drive input for the laser source.
The modulation controller may also include a digital modulation rail system having a digital modulation function generator capable of generating a digital waveform output and a programmably controllable digital modulation switch capable of selecting between the coherence control waveform output and a ground. In this case, each laser source card includes a second switch capable of selecting between the digital modulation rail system and ground. The second switch may be configured to provide a bypass of the gain block.
The source modulation system may be used in a method comprising the step of switching the laser source card between selected rails to accept a waveform output from the selected rail for use as laser source drive input.